Liquid crystal displays have been widely adapted for use in a number of products such as digital watches and clocks, laptop computers, and information and advertising display signs. Generally, the display includes a thin layer of liquid crystal material sandwiched between two transparent panels. An electrode array comprising a first set or plurality of parallel oriented electrode segments (row electrode segments) disposed on an inwardly facing side of one panel and a second set or plurality of parallel oriented electrode segments (column electrode segments) which are perpendicular to the row electrode segments disposed on an inwardly facing side of the other panel is provided. The row and column electrode segments are spaced apart by spacer material and the liquid crystal material is filled in the spaced apart region between the panels.
Display picture elements or pixels are defined by regions of liquid crystal material adjacent the intersections of aligned electrodes of the horizontal and vertical electrode segments of the electrode array. Upon application of a suitable electric field, a pixel will assume either a reflective or a non-reflective state. A pixel, pi,j, formed at the overlapping or intersection of the ith row electrode and the jth column electrode is subject to an electric field resulting from the potential difference between a voltage applied to the ith row electrode segment and a voltage applied to the jth column electrode segment.
Recent advances in liquid crystal material research has resulted in the discovery of bistable chiral nematic (also called cholesteric) liquid crystal materials. Cholesteric liquid crystal materials are able to maintain a given state (reflective or nonreflective) without the need for the constant application of an electric field. When data or an image displayed on a display is to be changed, some pixels will require a change in their state of reflectance while others will not. The display driver circuitry appropriately changes the electric field applied to those pixels whose reflectance states need to be changed in order to effect the desired change.
If the panel furthest from the viewer is painted with a black substrate, a pixel with a low reflectance will appear as a black area to the viewer. If the liquid crystal material has a light color appearance (such as yellow) in its highly reflective state, a pixel in a high reflectance state will appear to the viewer as a lighter colored area on the display.
Display driver circuitry is coupled to the vertical and horizontal electrodes of the electrode array. Operating under the control of a logic and control unit, the display driver circuitry energizes the row and column electrodes with appropriate voltage waveforms such that an appropriate voltage across each pixel is generated. The voltage across a pixel will either cause it to remain in its present state of reflectance or change its state of reflectance. The image generated by the display pixels may be modified by changing the state of selected pixels. In this way, text or image data can be presented for viewing.
In the invention disclosed in U.S. application Ser. No. 08/390,068, filed Feb. 17, 1995 and entitled "Dynamic Drive Method and Apparatus For a Bistable Liquid Crystal Display," a method and display driver circuitry for speeding the rate of updating a 1,000 row cholesteric liquid crystal display was disclosed. Application Ser. No. 08/390,068 is incorporated herein in its entirety by reference. An updating time of approximately one second for a 1000 row display was achieved. By simultaneously addressing multiple rows of the display with a pipelining scheme, the overall updating time for the display was kept at one second.
The dynamic drive disclosed in application Ser. No. 08/390,068 represents a significant reduction in update time for a 1,000 row liquid crystal display. However, while a one second updating time is suitable for display of static images, e.g., a map image, text material, etc., such an update time does not correspond to video display rate and is too slow to provide the appearance of continuous movement of moving images to the human eye in certain applications.